Apparently there was something more going on than just a 'noise'. During the last race of the season the engine started smoking in white and we had a film on the oil i.e. all the tell tale signs of a blow head gasket . When trying to find the origin the HGF it turned out that there were some surface issues on the block and the cylinder head and they for sure where not a tight fit anymore.
So the engine head and the engine blocks have been flattened and the liners of 2 cylinders needed some surface work (luckily no oversized cylinders needed) . Apparently the engine had been thermally overloaded in her former life and nobody cared to think about the consequences that could have... (we purchased it as overhauled old block new parts, new cyl. heads etcetera)

Not the best way of ending the season i must say ... glad we found it in time and now also 'mr squeaky' is gone!

Update : after 15 minutes of flawless running the engine again started to run badly /rough (missing at least 1 cylinder). Further investigation still showed a 'misfire' in cyl 1. The camshaft adjustment seems to be off and....how thing could get possibly worse, there is a cooling water leak on the top of the block but only when the engine is revved. Is there anyone on the forum who had similar issues with a df250 ?

And....engineless again! I couldnt mitigate with the seller ( who must be EX suzy dealer for a reason?) so here we are with a cracked shortblock and a rib which i could row if she wasnt 1500 kilo....no ribbing this year while saving up for a (new!!) engine next year. I am not sure how I will handle the guy who sold me this one (i have a few ideas but they do not guarantee results)